Project 4 focuses on the ability of circadian genes in the ventral tegmental area (VTA)-nucleus accumbens (NAc) circuit to regulate mood and motivational state. This is related to the knowledge that abnormal mood and other symptoms in many patients with depression show prominent circadian oscillations. We have demonstrated that NPAS2 (neuronal PAS domain protein 2), a transcription factor highly homologous to Clock, regulates an animal's responsiveness to emotional stimuli, including their activity in animal models of depression. Interestingly, NPAS2 is not expressed in the suprachiasmatic nucleus (SCN), a hypothalamic region important for circadian oscillations and their entrainment by environmental lighting. Rather, the highest expression of NPAS2 is seen in the NAc. Our hypothesis is that NPAS2[unreadable]acting within the NAc[unreadable] contributes to circadian variations in mood, locomotor activity, and motivation. In parallel, we have established a powerful influence of Clock itself on mood: mice lacking functional Clock protein exhibit a striking array of behavioral symptoms reminiscent of mania. This phenotype is reversed by lithium, and we have growing evidence that Clock action in the VTA per se is an important mediator of this behavioral phenotype. The goal of the proposed studies is to carry out a systematic evaluation of the role played by NPAS2, Clock, and related circadian gene products expressed in the VTA and NAc in the regulation of mood and motivation. This will be accomplished by use of mice with mutations in these various genes and of viral vectors that selectively manipulate the activity of the genes within the VTA-NAc. In addition, we will further establish the regulation of circadian gene expression in the VTA and NAc in response to chronic exposure to stress and antidepressant treatments. As well, we will identify and characterize the target genes through which NPAS2, Clock, and other circadian genes, acting as transcription factors, regulate the VTA-NAc circuit. We are also interested in cross talk between these circadian genes and CREB. CREB is known to regulate certain circadian genes in SCN, and we have found similar regulation in the VTA-NAc. Moreover, CREB, and circadian transcription factors, share some of the same target genes (e.g., cholecystokinin) in these brain reward regions.